1. Field of the Disclosure
Embodiments disclosed herein relate generally to catalysts and processes for the decomposition of ethers and alcohols to form olefins. More specifically, embodiments disclosed herein relate to hydrofluoric acid treated amorphous synthetic alumina-silica catalysts useful for the decomposition of ethers, such as alkyl tertiary-alkyl ethers, to form olefins, such as tertiary olefins.
2. Background
Ethers may be decomposed to corresponding olefins and alcohols in the presence of an acid catalyst. For example, alkyl tertiary-alkyl ethers, such as methyl tertiary-butyl ether (MTBE) or tertiary-amyl methyl ether (TAME) may be cracked to tertiary olefins and corresponding alcohols over an acidic solid catalyst. The resulting olefin products, isobutylene and isoamylenes, are important raw materials for various applications. Isobutylene, for example, is a raw material for production of synthetic rubber. Isoamylene is a raw material for various specialized applications, such as herbicides, flavors, fragrances, and a copolymerization agent, among others.
One major difficulty encountered in the catalytic cracking of alkyl tertiary-alkyl ethers is a relatively short catalyst cycle length, generally caused by polymer deposition on the catalyst due to the highly reactive nature of the tertiary olefins. Additionally, a very small amount of dienes or diene-precursors may be detrimental to catalyst longevity. Thus, dienes and diene-precursors in feed streams are typically removed by hydrogenation, distillation, or both.
Various natural clays are acidic materials that can serve as catalysts for acid catalyzed chemical reactions, such as the cracking of alkyl tertiary-alkyl ethers. Clays are naturally occurring crystalline phyllosilicate minerals (mostly aluminosilicates) with various impurities. Aluminum, magnesium, calcium, sodium, and the like, are important cationic components of layered silicate minerals. When these cationic species, especially mono and divalent cations, are removed by proper chemical means, Brönstead acidic sites are introduced to the clay materials, and they can serve as acidic catalysts for various chemical reactions.
Many clays have layered or ordered structures. For example, kaolin, montmorillonite, attapulgite, bentonite, beidellites, and other clays have layered structures. When such clays are properly treated, they become quite acidic. As a means to increase catalytic active sites, pillars may be introduced between the clay layers in addition to creating acidic sites. This pillaring technique multiplies the number of catalytic acidic sites.
Both acidic clays and synthetic aluminosilicates have been used as a matrix for producing fluid catalytic cracking (FCC) catalysts. Calcined kaolin has been utilized as a raw material in the synthesis of Y-zeolite in microsphere form. Additionally, dehydration of ethanol to ethylene over mixed oxides of alumina-silica and alumina catalysts was studied by J. Koubek et al., Proceedings of the 7th International Congress on Catalysis, Part B, 853, 1980.
U.S. Pat. No. 4,398,051 discloses obtaining high purity tertiary olefins, such as isobutylene, by decomposition of alkyl-tertiary-alkyl ethers over acidic catalysts. The catalyst used was an alumina compound supported on a carrier containing silicon dioxides. The alumina compound on a support is decomposed by calcining at high temperatures (750-1000° C.). The carriers included silica, montmorillonite, kaolinite, attapulgite, silica-zirconia, and others. However, no data regarding catalyst stability or catalyst deactivation is presented.
U.S. Pat. Nos. 5,043,518 and 4,691,073 disclose a process using clay catalysts for producing isoamylenes by cracking TAME over various natural clay catalysts, such as attapulgite clay, treated with an aqueous hydrofluoric acid (HF) solution. Benefits disclosed by treating the natural clays with HF include a higher activity and increased catalyst stability, which are measured in terms of cracking temperature required to maintain 95% conversion of TAME. This type of clay is also an effective catalyst for the production of isobutylene from MTBE. These patents also mention use of synthetic clays in passing, and do not present any data directed toward synthetic clays.
Commercially, attapulgite clay catalysts are sold as granules. Unfortunately, attapulgite clay catalysts do not have good physical integrity. Additionally, HF treatment further weakens the physical integrity, which is measured by attrition rate or crushing strength. As a result, the HF treated attapulgite clays must be handled with care, and the catalyst life is typically no more than 6 months.
Other downfalls of HF treated attapulgite clays include disposal costs. Such catalysts must be treated or otherwise rendered inert prior to disposal, adding to the cost of producing tertiary olefins. Additional costs are encountered by the large amount of HF required for the clay treating process, increasing raw material costs and creating a large amount of fluorinated waste solution. Although the HF treated attapulgite generates a lot of waste materials, the service time and deactivation are a significant improvement over untreated clay catalysts.
Another downside to use of HF treated clays includes increased production of byproducts, such as dimethyl ether (DME). Because of the higher catalytic activity of HF treated clays, they produce, in general, more DME than untreated clay catalysts. Catalyst deactivation typically results in a lower conversion of alkyl tertiary-alkyl ethers, such that the cracking temperature is raised to maintain a steady conversion as the catalyst deactivates. As treated attapulgite clay catalysts deactivate at a slower rate than untreated catalyst, the slower temperature ramping does not affect DME production as much as for untreated clay catalysts. Decreased DME production is a desired benefit not generally obtained with HF treated attapulgite clays.
U.S. Pat. No. 5,043,519 ('519) discloses a process for the production of tertiary olefins by decomposing alkyl tertiary-alkyl ethers in the presence of a catalyst containing 0.1 to 1.5 weight percent alumina on silica. As disclosed, at least 0.5 weight percent alumina is required for slower catalyst deactivation, and addition of small quantities of water into the feed stream may suppress DME formation. Nevertheless, the experimental results presented in Table 2 of the '519 patent shows more than about 1800 ppm DME by weight in the product stream at about 75% MTBE conversion, and more than 4500 ppm DME at a slightly higher conversion of 76.8%.
U.S. Pat. Nos. 4,880,787, 4,871,446, 4,888,103, and 4,324,698 each mention that amorphous silica-alumina catalysts are useful as cracking catalysts. However, as noted in U.S. Pat. No. 4,880,787, the major conventional cracking catalysts in use at that time (late 1980's, early 1990's), the same time period as the aforementioned U.S. Pat. No. 4,691,073, generally incorporated a large pore crystalline aluminosilicate. To the present inventor's knowledge, commercial synthetic silica-alumina catalysts for cracking remain to this day to be of a crystalline nature.
Accordingly, there exists a need for improved catalysts for the production of tertiary olefins. Such improved catalysts are desired to have one or more of an increased cycle length, good physical integrity, a low attrition rate, a high crushing strength, decreased production costs, and decreased disposal costs.